Easy Notes Of Data Communication and Computer Network unit-4 @Computer Diploma

Description

4.1 THE OSI MODEL: Layered Architecture, Encapsulation

The Open Systems Interconnection (OSI) Model is a conceptual framework used to describe the functions of a networking system. It organizes communication tasks into seven hierarchical layers.

• Layered Architecture: The model divides the complex process of network communication into smaller, more manageable sub-tasks, each handled by a specific layer. Each layer performs a specific function and relies on the layer below it to perform its functions, while providing services to the layer above it.

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• Encapsulation: This is the process where a protocol at a higher layer adds control information (like headers and footers) to the data before passing it to the layer immediately below it. The data unit at each layer is encapsulated within the header/footer of the next lower layer. This continues until the data reaches the Physical Layer and is sent as bits.

4.2 Layers in OSI Model (Functions of each layer)

The seven layers of the OSI model, from bottom to top, are:

4.3 TCP/IP Layers and their functions

The TCP/IP Model is a four-layer framework, much more practical and widely used than the OSI model. It generally maps to the OSI layers as follows:

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4.4 Protocols

4.5 Addressing

In a TCP/IP network, three main levels of addressing are used:

1. Physical Address (MAC Address):

   • Level: Data-Link Layer (Layer 2).

   • Format: A 48-bit (6-byte) address, usually written in hexadecimal (e.g., 00:1A:2B:3C:4D:5E).

   • Scope: Local (within a single network/LAN).

   • Purpose: Used for hop-to-hop delivery within a network. It is burned-in to the Network Interface Card (NIC).

2. Logical Address (IP Address):

   • Level: Network Layer (Layer 3).

   • Format: 32-bit (IPv4) or 128-bit (IPv6).

   • Scope: Universal (across the entire Internet).

   • Purpose: Used for source-to-destination delivery across the Internet. It defines the network and the specific host within that network.

3. Port Address:

   • Level: Transport Layer (Layer 4).

   • Format: A 16-bit number (0 to 65535).

   • Scope: Within a single host.

   • Purpose: Identifies a specific application process running on the host (e.g., port 80 for HTTP, port 23 for Telnet).

4.6 IP Address – Concept, Notation, Address Space

• Concept: An IP address is a unique numerical label assigned to every device connected to a computer network that uses the Internet Protocol for communication. It serves two main functions: host identification and location addressing.

• Notation (IPv4): Written in dotted-decimal notation (e.g., 192.168.1.1). It consists of four decimal numbers, each ranging from 0 to 255, separated by dots.

• Address Space (IPv4): Since an IPv4 address is 32 bits, the total address space is $2^{32}$, which is approximately $4.3$ billion unique addresses.

4.7 IPv4 Addressing: Classful and Classless Addressing, Subnet Mask, Supernetting, Subnetting

Classful Addressing

This was the original system, dividing the 32-bit space into different classes based on the first few bits:

Classless Addressing (CIDR)

Classless Inter-Domain Routing (CIDR) replaced Classful Addressing to solve the problem of address depletion and inefficient allocation.

• Notation: Addresses are written as IP Address / Prefix Length (e.g., 192.168.1.0/24). The prefix length (e.g., 24) indicates the number of bits used for the network ID.

Subnet Mask

A 32-bit number used to determine which part of an IP address belongs to the Network ID and which part belongs to the Host ID.

• It has a sequence of 1s for the Network ID followed by a sequence of 0s for the Host ID.

• Example: For a /24 prefix (Class C), the subnet mask is $11111111.11111111.11111111.00000000$, or 255.255.255.0 in dotted-decimal.

Subnetting

The process of dividing a single large network into smaller, multiple sub-networks (subnets). This is done by borrowing bits from the Host ID portion to create an extended Network ID (the Subnet ID). It increases the number of networks but decreases the number of hosts per network.

Super netting

The process of combining multiple small networks (e.g., multiple Class C networks) into a single larger network. This is done by borrowing bits from the Network ID and adding them to the Host ID. This is primarily used to improve routing efficiency (route aggregation).

4.8 IPv6 Addressing scheme and basic structure

IPv6 was developed to solve the IPv4 address depletion problem and introduce improvements in routing and security.

• Address Scheme: IPv6 addresses are 128 bits long.

  • Address Space: $2^{128}$ unique addresses, a virtually inexhaustible supply.

• Notation: Written in colon-hexadecimal notation.

  • The 128 bits are divided into eight 16-bit blocks, separated by colons (e.g., 2001:0db8:85a3:0000:0000:8a2e:0370:7334).

  • Simplification Rules:

    1. Leading zeros in any block can be omitted (e.g., 0db8 becomes db8).

    2. A single consecutive string of blocks containing only zeros can be replaced by a double colon (::).

• Basic Structure: An IPv6 address is generally divided into two main parts:

  1. Prefix (Network) (First 64 bits): Used for global routing and identifying the network/subnet.

  2. Interface ID (Host) (Last 64 bits): Used to identify the specific host within the subnet. It can be automatically generated using the device’s MAC address (EUI-64 format).